This invention relates to a method of and apparatus for repelling aquatic creatures such as sharks. The invention has particular application in the protection of swimmers or divers from sharks and will be described with reference to such an application by way of example.
Various shark repellant devices have been proposed over the years. A large number of these devices appear to rely on the electro-perception of sharks which is performed by the ampullae of Lorenzini in the shark's nose and head. These sensory organs have been shown, for instance, by Dr. Adrianus J Kalmijn (in research done for the Woods Hole Oceanographic Institution and the U.S. Office of Naval Research), to be extremely sensitive to electrical fields in sea water. Using fields decreasing to 5 nanovolts.cm.sup.-1 at distances 24 cm to 30 cm from the field source, Kalmijn was able to stimulate feeding attacks in response to electric fields simulating prey. It should be appreciated that this research was conducted against the background that the human body, especially when the skin is damaged, creates substantially stronger bioelectric fields, which some sharks in the ocean can detect from distances up to at least 1 m. The galvanic fields of metallic objects are usually even stronger, which, according to Dr. Kalmijn, could explain much of the aberrant behaviour of sharks in the presence of man and underwater gear.
It is possible, however, that electrical shark deterrents operating on larger field strengths exceed the sensitivity levels of the ampullae of Lorenzini to such an extent that these sensors are of no significance other than possibly to serve as electrical conductors of current. Substantial research will still have to be done in order to confirm these views, but it is clear that electrical deterrents need to rely on involuntary physical effects in order to be reliable and effective.
It appears unlikely that the ampullae of Lorenzini play any direct role in the shark's avoidance and startle reaction. It is possible, however, that these organs play an important passive role. The electrical resistance of living shark skin was measured at a number of places on the shark's body and also in the snout region where the largest number of the jelly-filled canals leading to the ampullae of Lorenzini, exist. It was found that shark's skin has a fairly low electrical resistance, but that on the snout, amidst the canals, the resistance was particularly low, some 5 or more times lower than the skin over the rest of the body. It appears that these low resistance canals may well offer a very low resistance pathway for the entrance of electrons and electrical current. It is felt that this may explain the fact that the shark's nerves are stimulated by aquatic electric fields at much lower electrical voltages than are those of humans.
U.S. Pat. Nos. 3,686,280 (Holt) and 3,164,172 (Hicks), describe shark repelling devices utilising pulse generators producing an electric field to divert sharks from the proximity of the generating apparatus. These early devices are fully traversed by U.S. Pat. Nos. 3,822,403 (Hicks), 4,667,431 (Mendicino) and 4,211,980 (Stowell).
Mendicino describes a device similar to a cattle prod or human crowd control tazor, but unlike these devices which are designed for mammals and which operate on high voltages (up to 40,000 V) and amperages in the mililamp range, the device described by Mendicino provides a 1-5 Amp, 300 V-1000 V charge in an attempt to repel sharks.
Stowell describes a method for repelling sharks by creating, about an electrode submerged in salt water, an electric field with a voltage gradient of sufficient magnitude to "overstimulate" (according to the patent) the nervous system of the shark. He describes a system which applies brief DC pulses to electrodes immersed in salt water with a relatively long delay between pulses (0.5 to 10 ms pulses spaced to a repetition rate of between 6 and 12 Hz).
Of the earlier patents referred to above, Hicks applies current pulses to electrodes to create an electric field between the electrodes at a low frequency of approximately 70 cycles per minute.
All of the devices described above utilise unidirectional current flows with the result that the device is able, in each case, to develop a positively charged region about its cathode. As is apparent from the research of Dr. Kalmijn and others, there is evidence to suggest that such a positively charged region serves as an attractant to sharks.
Like all animals, the shark's nervous system operates on the basis that an increased pulse repetition rate (rather than an increase in the pulse amplitude) is used to transmit intensity of sensation.
The shark's myo-neural physiology imposes a maximum effective nerve impulse frequency which is of the order of 50 Hz. The recovery inertia of the cell appears to prevent impulse frequencies in excess of this rate.
According to E. D. Smith, writing in the Transactions of the Institute of Electrical Engineers (volume 65 (8)--August 1974), this recovery or refractory period is of the order of 1 ms and is followed by a period of reduced sensitivity wherein a much increased field strength is required in order to produce a second action potential in the nerve. This leads him to conclude that the nerve fibre stimulation threshold displays a marked dependence on the shape of the stimulating pulse. According to these theories, any externally applied stimulating pulse should rise slowly in intensity with time--It should be semi-sinusoidal in shape. He goes on to suggest a shark repellant device with an optimum pulse (for a Dusky Shark (Charcharhinus obscurus)) of approximately 800 .mu.s in duration and a pulse repetition rate of between 1 and 500 Hz.
Smith differentiates between the pulse shapes and rates required to obtain electrotaxis (he attaches a unique interpretation to this term) and to produce a fright reaction in sharks. Smith then goes on to propose a most effective pulse repetition rate for electrotaxis as being:
15 Hz to 16 Hz PA1 using 800 .mu.s pulses of semi-sinusoidal shape PA1 at field intensities of between 5,5 V.m.sup.-1 to 10 V.m.sup.-1.